A key challenge in LIGA (described below) is the replication of multiple sacrificial molds for the electroplating of metal parts/structures. Many applications require metal parts for mechanical, electrical or other reasons. The ability to replicate electroplatable plastic molds would eliminate the need for repetitive synchrotron exposures, a costly and time-consuming step, as well as the subsequent development step.
The production of micro-scale metal parts via LIGA (German acronym for lithography, electroplating and molding) is a multi-step process requiring mask production, synchrotron exposure of the polymethylmethacrylate (PMMA) substrate (typically PMMA bonded to a metallized silicon wafer or a solid metal plate), development of the PMMA, electroplating to fill the cavities left within the PMMA mold, lapping and final dissolution of the remaining PMMA. Such technology is described in U.S. Pat. No. 5,378,583. A constraining step in this process is the requirement for access to one of the very limited number of synchrotron facilities. If the electroplatable PMMA (or other plastic) molds could be replicated without the need for repetitive synchrotron exposures, this would provide an enormous savings in time and cost in the production of larger quantities of the desired metal parts.
Numerous approaches to this replication problem have been explored at Kernforschungszentrum Karlsruhe GmbH.
U.S. Pat. No. 4,541,977, titled “Method For Producing Separating Nozzle Elements,” is directed to a very specific method for producing a complex multi-nozzle assembly including an array of micro-channels and nozzles held between top and bottom plates with corresponding inlets and outlets. This assembly is used for the separation of gaseous or vaporous mixtures. A process for the replication of the internal features is described which infuses a polymer into a master mold of the internal features contacting a metallic bottom plate. Upon removal of the master mold, the negative plastic mold remains adhered to the bottom plate due to the presence of dove-tailed inlets which lock onto the infused plastic. Subsequent electroplating of this structure provides a metal replicate of the original master features from which the plastic mold can then be removed.
U.S. Pat. No. 4,661,212, titled “Method For Producing A Plurality Of Plate Shaped Microstructured Metal Bodies,” provides a number of more general approaches for producing electroplatable plastic molds that rely on the use of metal or carbon filled PMMA formulations. Different methods are used depending on whether the features to be electroplated are contiguous or non-contiguous. Non-contiguous features require the casting of an unfilled non-conductive PMMA resin into the features of a master mold followed by a second overlay casting with a filled conductive PMMA. Upon demolding from the master, this filled PMMA provides a conductive and electroplatable base to which the unfilled PMMA features are bonded. One variation on this approach describes the prefabrication of a two-layer PMMA substrate in which one layer is unfilled and insulating and the bottom layer contains a conductive filler. This two-layer substrate is embossed with a master such that the features of the master penetrate through the insulating unfilled layer into the conductive filled layer. Such two-layer substrates are also used in other patents referenced below. The same patent describes another process suitable only for contiguous features in which the master mold is first dip-coated to apply a thin mold release layer to the feature tops and then similarly dip-coated in a conductive, filled PMMA formulation such that the feature tops only are coated. The wells between the features on this mold are then filled and covered over with an unfilled, non-conductive PMMA material. Upon demolding, the contiguous conducting path of the filled PMMA layer allows electroplating of the desired metal replicate.
U.S. Pat. No. 4,981,558 titled “Process For The Reproduction Of A Microstructured, Plate-Shaped Body,” discloses a process similar to that in U.S. Pat. No. 4,661,212 with the addition of the use of ultrasound to enhance penetration of the metal master through the insulating top layer and into the conducting PMMA bottom layer of a pre-formed two-layer PMMA substrate. Use of ultrasound permits the elimination of the heating and cooling steps normally involved in such embossing procedures.
U.S. Pat. No. 5,055,163 is titled “Process For Producing A Two-Dimensionally Extending Metallic Microstructure Body With A Multitude Of Minute Openings And A Tool Suitable For This Invention,” describes a more specific process similar to that in U.S. Pat. No. 4,661,212 in which a master mold with multiple tapered projections is embossed into a two-layer substrate in which the conducting lower layer might be a filled PMMA, another filled polymer or a low melting metal. The tapered feature tips facilitate penetration of the master features through the top layer and into the conducting layer of the substrate. The use of cylindrical master tools in a continuous process and the use of ultrasound are also described.
U.S. Pat. No. 5,073,237 titled “Method Of Making Molds For Electrodeposition Forming Of Microstructured Bodies,” discloses a method that overcomes some of the difficulties associated with the preceding processes by using a two-layer substrate that consists of a sputtered or vapor deposited film of metal or carbon on an insulating polymer base such as PMMA. During the standard embossing process, the metal film along the walls of the embossed features is stretched and disrupted to form a discontinuous and therefore non-conductive array of isolated spangles of the deposited film. The film in the bottom of the embossed features is not disrupted in this manner and provides a conductive contact for subsequent electroplating of the features. The features in this case must be contiguous, however.
It is important in the electroplating of micro-features with high aspect ratios that the walls of the electroplating mold be non-conductive. If the feature walls as well as the feature bases are conductive, the electroplating process will tend to close off the feature cavity before it has been completely plated up from the bottom. Such difficulties preclude the simple deposition of a metallic conducting film on the surface of a sacrificial plastic mold prior to electroplating or the use of conductive plastics in a standard embossing or injection molding process to form sacrificial molds. In the case of low aspect ratio features, either of the above options is readily applicable.
U.S. Pat. No. 5,162,078, titled “Method Of Producing Microstructured Metallic Bodies,” is directed to the removal by reactive ion etching of residual polymeric films in the bottom of plastic mold cavities, which would prevent electroplating on the conductive base supporting those features. Such residues are a potential problem when embossing through the two-layer substrates described in many of the above patents. The reactive ion etch is directed perpendicularly to the surface of the base plate to avoid degradation of the plastic features.
U.S. Pat. No. 5,676,983 titled “Tool For Making A Microstructured Plastic Mold From Which Structures Can Be Formed Galvanically,” and U.S. Pat. No. 5,795,519 titled “Process Of Making A Microstructured Plastic Mold,” again utilize a two-layer substrate but provide an embossing master tool in which the features have smooth walls but the top surfaces of the features possess rough surfaces having points and ridges adapted to penetrate into the electrically insulating layer. This enhanced penetration allows the embossing tool to more efficiently expose the electrically insulating layer at the bottom of the embossed cavities.
None of the above processes provide a simple and versatile method of replicating either contiguous or non-contiguous features in a sacrificial plastic mold. Many require the pre-fabrication of specific plastic substrates, which contain a conducting layer adhered to a non-conducting layer with precise height requirements. Various techniques have been used to insure penetration of the tooled embossing features through the non-conducting layer into the conducting layer. Some of these require the fabrication of special embossing tools with sharpened or roughened features. Some of the above techniques are also applicable only to contiguous features, a major limitation. There remains a need in the micro-fabrication art for a general method capable of replicating metal structures and parts now made by processes such as the described LIGA technology.